Measurement of caustic and acidic reactive gases, such as for example hydrogen sulfide (H2S), hydrogen chloride (HCl), hydrogen fluoride (HF), hydrogen cyanide (HCN), hydrogen bromide (HBr), arsine (AsH3), phosphine (PH3), and ammonia (NH3), in gas streams containing a complex mixture of potentially interfering analytes presents a number of difficulties. Accurate characterization of such analytes can be quite important in a wide range of applications, including but not limited to petroleum product processing, extraction, transportation, and combustion, and numerous industrial processes.
The reactive gases discussed herein, as well as other gases with similar properties, can present significant environmental and human safety hazards. Hydrogen sulfide, for example, is a gas with a characteristic “rotten egg” odor that is highly flammable in air (in a concentration range of approximately 4.3% to 45% by volume). OSHA (Occupational Safety and Health Administration) regulations consider concentrations of 100 ppm H2S “immediately dangerous to life and health”, while concentrations greater than 700 ppm lead to immediate death. NIOSH (National Institute for Occupational Health) recommends the maximum exposure of humans to H2S to not exceed 10 ppmv for 10 minutes. HF and HBr have OSHA permissible exposure limits of 3 ppmv while the limits for HCN, PH3, AsH3, and NH3 are 10 ppmv, 0.3 ppmv, 0.05 ppmv, and 50 ppmv, respectively. These gases are common byproducts of many industrial petrochemical processes including ethylene, propylene, Teflon™, polyvinyl chloride (PVC), nylon, viscose rayon production, and rubber production, as well as of de-sulfurization of natural gas and crude oil. Trace amounts as low as several ppbv to a few ppmv of these chemicals can seriously impede the respective chemical processes and lead to defective plastics and other products.
Hydrogen sulfide also occurs naturally in natural gas, oil, sewage and landfills. Its release to the environment, or incineration forming and releasing SO2, even in small quantities, needs to be prevented. This requires sensitive detection of H2S in pipelines, petrochemical processes and in and around facilities dealing with this gas. Rising world wide energy demand and energy prices have driven an increasing use of crude oil and natural gas with very high H2S concentrations which places ever rising demands on H2S removal technologies and reliable, accurate, and sensitive detection of H2S.
Natural gas is an important energy source for industry and personal homes because of its low cost and widespread availability. Unpurified natural gas can contain up to 60% hydrogen sulfide. Even less than 4 ppmv amounts of H2S can lead to corrosion of delivery pipelines, over time, potentially resulting in serious explosion and leakage hazards while necessitating costly replacement of segments of the pipeline. For reference, downtime for natural gas pipelines can cost upwards of several thousand dollars per second. Hydrogen sulfide must be removed from natural gas to prevent pipeline corrosion and the emission of another toxic gas, SO2, which is created during burning. Sensitive, real time detection of hydrogen sulfide in natural gas is becoming increasingly more important to facilitate limiting concentrations to below the 4 ppm tariff level.
Additional concerns with H2S in gas streams can arise due to its tendency to degrade or poison catalysts in chemical processes. Such poisoning can occur due to deposition of surface sulfur on the metal components of a catalyst body and the substitution of sulfur ions for oxygen ions near the surface of metal oxides. In other cases, sulfur can chemically substitute for oxygen in the surface oxides of a catalyst, thereby creating metal sulfides with reduced activity.
Refinery fuel gas is an important energy source for petrochemical processing plants, generating energy from combustible waste gases which occur as byproduct of petrochemical processes, including production of ethylene, propylene and iso-butane, which are the fundamental building blocks for all types of plastics. U.S. Environmental Protection Agency regulations limit hydrogen sulfide levels in fuel gas to less than 160 ppm for emission of H2S and SO2. Also, the presence of NH3 in fuel gas can lead to formation and emission of environmentally incompatible nitrous oxides during combustion.
Conventional techniques for measuring reactive gases rely primarily on the use of chemical sensors such as lead acetate tape, broadband non dispersive UV photometry, gas chromatographs (GC) or small surface area electro-chemical sensors, such as metal-oxide semiconductors. These techniques have generally proven unsuitable for on-line or at-line real time process control and real time hazard prevention. Drifting calibration, slowness of measurement, sensor saturation, long recovery times after sensor saturation and sensor element degradation from contaminants in the background gas typically lead to erroneous readings and can lead to harmful emissions or a failure to detect hazardous or unacceptably high concentrations of reactive gases to go undetected. Such conventional sensors tend to be quite maintenance intensive, requiring frequent replacement of costly consumables such as lamps, GC columns, carrier gas for GCs, lead acetate tape or aqueous ammonia solution. In addition, lead acetate tape analyzers create significant amounts of hazardous, lead containing waste. Analyzers relying upon indirect, UV detection of H2S create di-ammonium sulfide, (NH4)2S, waste which can easily release H2S again. Additional limitations can arise from electrochemical sensors' abilities to interact with only a very small portion of the gas environment, directly at the location of the sensor element itself. This can potentially lead to mischaracterization of harmful concentrations, particularly in a less than ideally mixed gas stream. Conventional sensors also tend to be highly sensitive to condensable contaminants in the background gas stream which can cause erroneous readings and earlier than expected sensor failure.
Gas chromatographs (GCs) are being used, generally delivering accurate results albeit at very high initial, system and infra structure cost, high consumables cost for carrier gas and separation columns and high ongoing maintenance cost, providing measurement cycle times in the order of several minutes. This approach is generally too slow and does not allow real time process control feedback and detection that can effectively be used to prevent or reduce harmful H2S concentrations, particularly in gas streams with rapidly varying compositions.
Attempts have been made to measure H2S and other reactive gas concentrations by means of ultraviolet light absorption spectroscopy, using a spectrally broad ultraviolet lamp and a diffraction grating. The continuum-like absorption spectrum of, for example, H2S in the ultraviolet can cause erroneous, ambiguous readings and limited repeatability of measurement, especially in interfering background gas streams, making it unsuitable for precise, real time process control and hazard prevention. To overcome this potential ambiguity of direct UV based H2S concentration measurement, some non dispersive, broadband UV instruments attempt to indirectly determine H2S concentration by converting H2S into (NH4)2S and photometrically measuring (NH4)2S. This technique heavily relies upon the assumption that the chemical reaction to (NH4)2S is complete, without aging effects and temperature influence. Another instrument combines the non-dispersive UV absorption technique with gas chromatography columns to separate H2S from the remainder of the gas stream, suffering from the same inadequacies as a GC.
HCl, HF, HBr, AsH3, and NH3 are also common in a variety of industrial and petrochemical processes and applications, either as chemical process byproducts or in feed streams. HCl, HBr, and HF are corrosive gases, particularly in the presence of any moisture. Among other possible sources, they can be used in various applications for production of plastics and polymers, including PVC, Teflon and nylon, and can also be byproducts of petroleum cracking, especially in alkylation processes. Ammonia is a caustic gas with a characteristic pungent odor that often serves as a precursor to foodstuffs and fertilizers and some pharmaceuticals. Arsine and phosphine have proven detrimental to polymerization reactions, which create the plastic feed stock, even in very low ppb level concentrations. Various wet chemical detection techniques or specialized gas chromatographs are available for quantifying HCl, HBr, HF, HCN, AsH3, PH3, and NH3, but like H2S, real time, accurate, robust, and low maintenance detection and quantification methods for use in industrial processes have been lacking.